In plant facilities for desalination of seawater by reverse osmosis membrane methods, it is common practice to employ a method of introducing seawater or the like into a pressure vessel (internal pressure vessel) loaded with a hollow fiber membrane or spiral membrane, and applying pressure to separate desalinated water from concentrated water (concentrated seawater).
In the course of such a reverse osmosis membrane method, the interior of the pressure vessel is continuously subjected to internal pressure of 800-1,200 PSI (5.5-8.3 MPa), which is 5.5-8.3 times the 1 MPA of compressed air generated by a typical compressor. Therefore, exceedingly high pressure resistance is required of pressure vessels. Moreover, because pressure vessels come into contact with seawater and concentrated seawater, typical practice is to employ fiber-reinforced plastics (FRP), which are endowed with good corrosion resistance and pressure resistance.
Filament winding (FW), by which high pressure resistant qualities can be designed, has been adopted as the method for molding FRP.
It is widely known that in general, the mechanical characteristics of FRP are greatly affected by the orientation of the fibers thereof. For example, while a high level of pressure resistance is obtained when tensile load is applied in the lengthwise direction (X direction) of the fibers of an FRP, a high level of pressure resistance is not obtained when tensile load is applied in the width direction (Y direction) of the fibers of the FRP. This characteristic is termed anisotropy, and differs greatly from isotropic materials such as metals.
In order for an FRP produced by FW to utilize isotropy and retain internal pressure, it is preferable during fabrication of the pipe body for the angle during winding of the filaments (glass fibers) onto a mandrel to be set to 55 degrees (50 degrees to 60 degrees, depending on the method of calculation) with respect to the direction of the axial center of the mandrel. In FW, such a fiber layer in which the winding angle of the fibers is inclined with respect to the direction of the axial center is termed a helical layer (see FIG. 1. FIG. 1 shows an example of the case of a helical angle of 55 degrees).
In order to avoid a situation in which there is pressure resistance in the circumferential direction only, and no pressure resistance in the lengthwise direction (for example, when pressure is applied to an FRP pressure vessel, no change is observed in the circumferential direction, but stretching is observed in the lengthwise direction. This can occur, for example, in cases in which the winding angle is close to 90° C.; see FIG. 2), or in which there is pressure resistance in the lengthwise direction only, and no pressure resistance in the circumferential direction (for example, when pressure is applied to an FRP pressure vessel, no change is observed in the lengthwise direction, but the inside diameter stretches in the circumferential direction. This can occur, for example, in cases in which the winding angle is close to 0° C.; see FIG. 3), it is preferable for the winding angle of the fibers that form the helical layer to be one that affords a balance between pressure resistance with respect to pressure in the circumferential direction of the pressure vessel, and pressure resistance in the lengthwise direction (direction of the axial center).
In pressure vessels produced by simple FW, the pipe body is fabricated by winding fibers into layers (helical layers) at this angle of 55 degrees. When internal pressure is applied to this pipe body, the occluding covers 2 at either end of the pipe body, which have been provided at either end of the pressure vessel, are subjected to large loads (forces tending to push the occluding covers towards the outside of the pipe body due to internal pressure).
When internal pressure is applied to a pressure vessel, the force thereof is transmitted to retainer rings 53 that fasten together the occluding covers 52 and the pipe body 51 at either end of the pipe body 51 (see FIG. 4). Because the vector of this force is directed parallel to the direction of the axial center of the pipe body, the force acts in such a way that FRP layers at the front surface of the retainer rings of the pipe body 51 are pushed towards the outside of the pipe body by the retainer rings 53, in a direction parallel to the axial center. This force acts as interlayer shear force in the circumferential direction on the respective layers, in each of FRP layers wound into layers at 55 degrees and FRP layers wound into layers at 90 degrees, resulting in rupture of the FRP layers at the front surface of the retainer rings (see FIG. 5).
In the case of a pressure vessel of the above structure, in order to maintain high pressures of 800-1,200 PSI (5.5-8.3 MPa), there could be adopted either a method of increasing the thickness of the layered wound layers (increased rigidity), or adopting a greater distance from the retainer rings to the ends of the pipe body to increase the shear distance (increased shear force); however, either of these will result in a heavier pressure vessel.
In a pressure vessel, the barrel section of the pipe body 51 is rigid, and in many cases, the pressure resistance thereof to pressure is dependent upon the pressure resistance of the FRP layers surrounding the retainer rings.
According to American Society of Mechanical Engineers (ASME) Section X, which is the technical standard in the field, the internal pressure-induced rupture pressure (safety factor) is specified as being 6 or more times higher than the operating pressure. In an FRP pressure vessel, design of fiber orientation (design of anisotropy) in this section has a large effect on whether a pressure vessel is good or bad.
When pressure is applied to a pressure vessel, there is accompanying deformation in the circumferential direction in the surrounding areas of the occluding covers 52, and therefore in many instances, wound fiber layers (hoop layers) at 90 degrees with respect to the axial center are furnished only in the surrounding areas of the retainer rings, increasing the thickness of the pipe body in these sections. For this reason, most FRP pressure vessels for seawater desalination purposes are larger in diameter at both ends, like a dumbbell.
In particular, as the force arising from tensile rupture of FRP layers affords considerably greater pressure resistance as compared with the force arising from shear rupture of FRP layers, a pressure vessel employing a FW method like that disclosed, for example, in Patent Document 1 has been proposed, in order to meet the safety factor requirements in the aforementioned ASME Section X. Patent Document 1 discloses a structure furnished, in zones at either end of an inner ring, with hoop layers wound in the circumferential direction about a mandrel, and in other zones with helical layers wound at an acute angle with respect to the axial center of the mandrel (see FIG. 1 of Patent Document 1) (additional hoop layers being furnished, only at both ends of the aforementioned helical layer).